The present invention relates to the field of hydromechanical transmissions utilizing both hydraulic and mechanical transmission paths between a rotating input and a rotating output.
A hydromechanical transmission is an infinitely-variable transmission and thus, may be used with a fixed or variable speed input rotation to provide a fixed or variable speed output rotation. This characteristic is useful for increasing the operating efficiency of both fixed and vehicular applications, since a power source may be operated near its most efficient speed, and the hydromechanical transmission controlled to provide a required range of output rotations. For instance, a turbine engine has a rather narrow range of peak efficiency, while an internal-combustion engine has a somewhat broader but still relatively narrow peak efficiency operating speed.
Although having a slightly lower efficiency than that of a purely mechanical transmission, due to the addition of hydraulic losses, hydromechanical transmissions have been thought desirable for over the road vehicular applications, due to the possibility of operating the prime mover or vehicle engine at a relatively constant speed near its most efficient speed, and have been thought desirable in off-road vehicular applications, since the continuous variability of the hydromechanical transmission provides smooth over-the-ground motion. This characteristic is desirable in such applications as heavy dump trucks, which often operate in soft soil, where an interruption in torque may cause the vehicle to sink into the surface, and in road graders, where an interruption in torque may provide an uneven road bed, and in farm implements, where uneven torque may cause the breakage of agricultural implements.
However, while hydromechanical transmissions have been successfully used in the past, they have been relatively intolerant of wide variations in input speed, being two or three-mode transmissions. These transmissions utilized as many as four gear sets to provide three modes of operation and did not provide an overdrive range. Further, such transmissions were not lockable at a direct drive ratio, to provide the efficiency of a conventional transmission, or could not be operated near this point without causing substantial changes in torque, such as the jolt provided by certain recent automotive transmissions. Such an operating characteristic is most unsuitable when operating heavy equipment, particularly when operated under conditions of uncertain traction or in soil of unpredictable characteristics.
Examples of prior art hydromechanical transmissions are disclosed in U.S. Pat. Nos. 3,714,845; 3,979,972; 4,306,467; and 4,341,131.